For 6.2.12 ~ 8.4.1, in shell execute:
sed -i.backup -z "s/res === null || res === undefined || \!res || res\n\t\t\t.data.status.toLowerCase() \!== 'active'/false/g" /usr/share/javascript/proxmox-widget-toolkit/proxmoxlib.js && systemctl restart pveproxy.service
For 8.4.5 ~
cat <<'EOF' >/etc/apt/apt.conf.d/no-nag-script
DPkg::Post-Invoke { "dpkg -V proxmox-widget-toolkit | grep -q '/proxmoxlib\.js$'; if [ $? -eq 1 ]; then { echo 'Removing subscription nag from UI...'; sed -i '/.*data\.status.*active/{s/!//;s/active/NoMoreNagging/}' /usr/share/javascript/proxmox-widget-toolkit/proxmoxlib.js; }; fi"; };
EOF
apt --reinstall install proxmox-widget-toolkit && service pveproxy restart
*You may need to clear browser’s cache for the pop-up to disappear.
Source: https://github.com/community-scripts/ProxmoxVE/issues/6078